This invention relates to a vehicle suspension comprising leaf springs, in particular a suspension for a rigid axle with improved roll steer behaviour.
In vehicle suspensions where a rigid axle is located and controlled by leaf springs, many compromises have to be made. Such compromises can adversely reflect on the suspension performance under various loading conditions to which it is subjected during the operation of the vehicle. Examples of such loading condition is vehicle roll about the longitudinal roll centre which occurs when the vehicle is negotiating a bend or is subjected to forces induced by strong winds in the transverse direction. A further loading is termed “bump steer”, which occurs when a wheel on one side of the vehicle is deflected upwards by an obstacle on the road surface. This type of loading affects the roll steer behaviour of the vehicle and the effect can be reduced by improving the auxiliary roll stiffness component of the suspension. The effect of spring stiffness on roll motion is combined with auxiliary roll stiffness to give the total roll stiffness. The auxiliary roll stiffness is a component of the total roll stiffness derived from suspension components other than the springs themselves (trailing arm systems, anti-roll bars, etc.
Some attempts to increase the auxiliary roll stiffness component have been made previous to the invention described below. Vehicle manufacturers have achieved limited success by increasing the longitudinal asymmetry of the spacing of the axle upon the springs. Specifically, by locating the axle at a point between of the midpoint of the leaf springs and their direct connection to the vehicle frame via the spring hanger, and by increasing the torsional stiffness of the axle, approximately ten percent gains have been made in the auxiliary roll stiffness component. This is due to a correlating increase in both of the two subcomponents of the auxiliary roll stiffness component, leaf twist and axle torsion.
The increase in the leaf twist subcomponent can be visualized as follows. As the vehicle negotiates a change in direction, the springs are loaded asymmetrically in the lateral direction. As a result the vehicle body leans. This produces an angularity between the axle and the vehicle frame in the lateral direction, with the outer spring compressed to a greater extent, and the inner spring relieved to some extent. The springs become the compliant member which accepts this angular difference. That is, they are twisted slightly along their length. Because the leaf springs are affixed to the chassis at their extremities, the twist occurs between the front spring eye and the mid-point axle attachment, and between the rear spring eye and the midpoint axle attachment. The ability of each spring half-portion of the overall length to resist this twist is a function of the shear modulus of the material, its polar moment of inertia, and the length of that half-portion. Because the torsional spring rate of the spring half-portion of the overall length is a function of the inverse of the length of that half-portion, the rate at which the torsional spring rate increases for the spring half-portion which is made shorter by longitudinal asymmetry becomes rapidly greater than the rate at which the torsional spring rate decreases for the spring half-portion which is made longer by that same longitudinal asymmetry. Because the direct connection between the spring and the vehicle frame via the spring hanger is generally more rigid than the connection via the spring shackle, or the member which compensates for the variation of the spring length upon deflection, that is generally the end of the spring toward which the axle is located.
The increase in the axle torsion subcomponent of auxiliary roll stiffness can be visualized as follows. As the vehicle leans, the outer spring is compressed to a greater extent, and the inner spring is relieved to some extent, as mentioned previously. As a leaf spring is compressed, it generally flattens in the case of a parabolic spring, or becomes invertedly parabolic in the case of a flat spring. It also changes in distance between the spring eyes, which explains the need for the spring shackle mentioned previously. At some point at or near its mid-point, a tangent drawn to the spring at that point remains at a fairly constant angle relative to the longitudinal axis of the vehicle throughout deflection of the spring. Forward and rearward of that theoretical midpoint, the angle between a tangent drawn to the spring and the longitudinal axis of the vehicle will change throughout deflection of the spring. By attaching the axle to a point other than that theoretical midpoint, generally in the direction from the theoretical midpoint toward the direct connection between the spring and the vehicle frame via the spring hanger, torsion is introduced to the axle, due to the fact that the inner and outer springs are deflecting in opposite directions resulting in opposite changes in the angularity between the tangents drawn to the springs and the longitudinal axis of the vehicle. By also increasing the torsional rigidity of the axle, the axle torsion subcomponent of auxiliary roll stiffness is increased.
An example of a conventional vehicle suspension arrangement is shown in FIGS. 1a and 1b, which show a schematic side view of a pair of prior art front wheel suspensions. In these figures the same reference numbers will be used for denoting the same component parts. The vehicle suspension in FIG. 1a comprises a pair of leaf springs 11 (one shown) arranged to extend longitudinally on opposed sides of a vehicle frame 12. Each leaf spring has a first end pivotally connected to the vehicle with a first bracket or spring hanger 13 attached rigidly to each leaf spring frame at a first position. A second end of the leaf spring is connected to the vehicle frame with second bracket or spring hanger 14a and a spring shackle 15, in order to compensate for length changes of the leaf spring under load conditions. A rigid axle 16 extends transversely of the vehicle frame, which axle is mounted to each leaf spring by means of a further bracket at a position between the respective first and second ends. The axle 16 is provided with an axle plate 19 which rests against the leaf spring 11. The leaf spring 11 is attached to the respective spring hanger 13, 14a at a front and a rear leaf spring end eye 17, 18, respectively. In this example the axle is a front steering axle A damper means (not shown) such as an air spring is mounted between the axle and the vehicle frame at or adjacent the axle.
FIG. 1a shows a leaf spring arrangement for a normal steered vehicle, having negligible axle steer. As the leaf spring 11 is deflected under load a point P, located at the centre of the axle plate 19 and intersected by a normal plane through the axle plate at this point, will follow an arc determined by the location of the so-called Ross point Rp. As the load on the spring is increased and decreased, said point P will move between an upper point P1 and a lower point P2. As can be seen from FIG. 1a, an imaginary line through the upper and lower points P1, P2 is substantially vertical, resulting in a neutral steered suspension with virtually no axle steer.
For steered rigid axles the normal design is to create an understeer behaviour, which requires a relatively large rearward inclination for both the Ross line and the datum line. The datum line is an imaginary reference line between the front and rear leaf spring end eyes. An example of this is shown in FIG. 1 b, in which the vehicle suspension comprises a pair of leaf springs 11 arranged to extend longitudinally on opposed sides of a vehicle frame 12. Each leaf spring has a first end pivotally connected to the vehicle with a first bracket or spring hanger 13 attached rigidly to each leaf spring frame at a first position. A second end of the leaf spring is connected to the vehicle frame with an extended spring hanger 14b and a spring shackle 15. A rigid axle 16 extends transversely of the vehicle frame, which axle is mounted to each leaf spring by means of a further bracket at a position between the respective first and second ends. The axle 16 is provided with an axle plate 19 which rests against the leaf spring 11. The leaf spring 11 is attached to the respective spring hanger 13, 14b at a front and a rear leaf spring end eye 17, 18, respectively. The extended spring hanger 14b causes the required rearward inclination for both the Ross line and the datum line.
The datum line is an imaginary reference line between the front and rear leaf spring end eyes 17, 18. It is generally known in the art that leaf springs under load will arc about at imaginary point in space which is called the “Ross point” which in turn determines the so-called “Ross line”. The Ross point is indicated at Rp and the Ross line is indicated at R|_ the FIGS. 1a and 1b. It is further known that the Ross line should be as flat and long as possible and the Ross line geometry should match the drag link geometry of the vehicle steering linkage (not shown). The drag link connects the pitman arm and the idler arm in a conventional steering linkage.
FIG. 1b shows a leaf spring arrangement for an understeered vehicle, with axle steer. As the leaf spring 11 is deflected under load a point P will follow an arc determined by the location of the Ross point Rp as described above. As the load on the spring is increased and decreased, said point P will move between an upper point Pi and a lower point P2. As can be seen from FIG. 1b, an imaginary line through the upper and lower points P1, P2 is angled in an upwards and rearwards direction. The arrangement of the leaf spring 11 in FIG. 1b causes a longitudinal difference x between vertical lines through the upper and lower points P1, P2, resulting in an understeered suspension with axle steer in the X-Y plane. In the subsequent text, the x-axis is located in the longitudinal direction of the vehicle, the y-axis is located in the transverse direction of the vehicle and the z-axis is in the vertical direction, at right angles to the X-Y plane.
The Ross line and datum line angle in FIG. 1b will result in an axle steer, which causes an axle rotation in the z-direction, as indicated in FIG. 2. FIG. 2 shows a schematic plan view of the suspension indicated in FIG. 1b. FIG. 2 shows a left and right leaf spring 21, 22, which are connected by a rigid steered axle 23, corresponding to the axle 11 in FIG. 1b in its loaded position. The figure also shows the steered axle 23, indicated in dashed lines, in the position when the load on the spring is decreased, and the near side of the axle 23 is at the lower point P2. This loading situation occurs when a driver steering input in the steering direction S causes the vehicle to roll to the right during a left hand turn. The figure also shows a steering box 25 which has a pitman arm (not shown) connected to a steering rod 26 and a spindle arm 27 for controlling the steerable wheel 28. The steering box 25 is attached to the frame, which has been left out for clarity. During a left hand turn the load on the spring will cause the axle to be displaced from the position indicated by the axle 23 to the position indicated by the axle 24, resulting in a displacement of the wheel centre 29. As the position of the steering rod 26 is controlled by the steering box 25, which is fixed, the displacement of the wheel centre 29 will cause a rotation of the wheel 28 in a direction contrary the steering input, resulting in understeer.
However, in order to achieve this axle steer the rear spring bracket 14b requires high stiffness due to the relatively large vertical offset from the chassis frame. Such an arrangement is space consuming, heavy and expensive.
The design described above, with a rearward angled leaf spring and with the Ross and datum lines close to parallel, will give very little longitudinal motion in the direction of the datum line. This reduces the forces in the spring and axle system so that the axle steer will have a direct understeer influence on the total steering behaviour. In this context, the total steering is the combined steering effect created by driver steering input, understeer and roll steer. When the steering rod moves the spindle arm in a longitudinal direction, the axle steer will cause an understeer effect during a roll motion of the vehicle frame.
FIG. 3 shows a side view of an alternative prior art vehicle suspension where the angle between the Ross line RL and the datum line DL is large enough to achieve a desired axle steer behaviour. The prior art vehicle suspension in FIG. 3 comprises a pair of left and right leaf springs 31a, 31b arranged to extend longitudinally on opposed sides of a vehicle frame 32a, 32b. Each leaf spring has a first end pivotally connected to the vehicle with a first bracket or spring hanger 33a, 33b attached rigidly to each leaf spring frame at a first position. A second end of the leaf spring is connected to the vehicle frame with second bracket or spring hanger 34a, 34b and a spring shackle 35a, 35b, in order to compensate for length changes of the leaf spring under load conditions. A rigid axle 36 extends transversely of the vehicle frame, which axle is mounted to each leaf spring by means of a further bracket at a position between the respective first and second ends. The axle 36 is provided with an axle plate 39a, 39b which rests against the respective leaf spring 31a, 31b. The leaf springs 31a, 31b is attached to the respective spring hanger 33a, 33b; 34a, 34b at a front and a rear leaf spring end eye 17a, 17b; 18a, 18b, respectively.
The steered axle 36, indicated in FIG. 4, corresponds to the axle position when the load on the left hand spring is decreased, and the end of the axle 36 at the near side is at the lower point P2. Similarly, the end of the axle 36 at the far side is at the upper point Pi. This loading situation occurs when a driver steeling input in the steering direction S causes the vehicle to roll to the right during a left hand turn.
When the angle between the Ross line RL and the datum line DL is large enough to achieve a desired axle steer behaviour due to the displacement of the wheel centres 41a, 41b, as indicated by a schematically indicated axle position in FIG. 4. However, the result will be large reaction forces between the two leaf springs 11a, 11b. This will cause a twisting deformation of the axle 36 and the leaf springs 11a, 11b, as schematically indicated in FIG. 5. The deformation of the axle 36 will displace the wheel centres 41a, 41b in a way that reduces the desired understeer effect on the total steering. FIG. 5 shows the actual position of the axle 36 as well as the desired position of the axle (dashed lines) from FIG. 4.
The solution according to the invention aims to provide an improved leaf spring arrangement that overcomes the above problems.